German patent publication 3,444,313 discloses a suspension mechanism for a surgical microscope. This suspension mechanism includes a spring for generating a linear force which compensates a torque caused by gravitational forces acting on the microscope. With a Bowden cable, the spring for generating the linear force is coupled to a gear wheel which is rigidly connected to the axis of rotation of the microscope. The suspension mechanism further includes adjusting means in the form of a gear shaft and a worm gear shaft which allow to adjust the amplitude and the phase of the compensating torque with respect to the torque caused by the gravitational forces.
From U.S. Pat. No. 5,492,296, a suspension mechanism for a surgical microscope is known, which comprises a spiral spring compensating a torque generated by gravitational forces. The force of the spiral spring can be set by an adjustment screw.
U.S. Pat. No. 5,213,293 discloses the use of gas pressure springs in a link-parallelogram for generating a force which counteracts the gravitational forces acting on a microscope stand.